using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class TEST{
	static void Main(){
		Sol mySol =new Sol();
		mySol.Solve();
	}
}

class Sol{
	public void Solve(){
		N = S.Length;
		double ret = 0;
		for(int i=1;i<=26;i++){
			ret += calc(i);
		}
		Console.WriteLine(ret);
	}
	
	double calc(int len){
		
		var dl = new int[256];
		int l = 0, lcnt = 0;
		var dr = new int[256];
		int r = 0, rcnt = 0;
		double cnt = 0;
		for(int i=0;i<N;i++){
			if(l < i) l = i;
			while(l < N){
				if(dl[S[l]] > 0){
					dl[S[l]]++;
					l++;
					continue;
				}
				if(lcnt < len - 1){
					dl[S[l]] = 1;
					l++;
					lcnt++;
					continue;
				}
				break;
			}
			while(r < N){
				if(dr[S[r]] > 0){
					dr[S[r]]++;
					r++;
					continue;
				}
				if(rcnt < len){
					dr[S[r]] = 1;
					r++;
					rcnt++;
					continue;
				}
				break;
			}
			if(rcnt == len){
				cnt += r - l;
			}
			if(dl[S[i]] > 0){
				dl[S[i]]--;
				if(dl[S[i]] == 0) lcnt--;
			}
			if(dr[S[i]] > 0){
				dr[S[i]]--;
				if(dr[S[i]] == 0) rcnt--;
			}
		}
		//Console.WriteLine("len:{0}, cnt:{1}",len, cnt);
		
		double tot = (double) N * (double) (N + 1) / 2.0;
		return (double) len * cnt / tot;
		
	}
	
	
	
	
	int N;
	String S;
	public Sol(){
		S = rs();
		
	}

	static String rs(){return Console.ReadLine();}
	static int ri(){return int.Parse(Console.ReadLine());}
	static long rl(){return long.Parse(Console.ReadLine());}
	static double rd(){return double.Parse(Console.ReadLine());}
	static String[] rsa(char sep=' '){return Console.ReadLine().Split(sep);}
	static int[] ria(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>int.Parse(e));}
	static long[] rla(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>long.Parse(e));}
	static double[] rda(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>double.Parse(e));}
}